Processes during charge and discharge of negative battery plates
During LAB operation the active mass of negative plates often expands considerably and part of it sheds off. The phenomenon of plate expansion during cycling is related to the changes in the organization of the active mass structure.
Processes causing plate pulsation. During discharge Pb dendrites (energetic structure) are oxidized to PbSO4 crystals. The latter are incorporated into the skeleton structure. Since the resulting product of the reaction (PbSO4) has a greater volume than the initial substance (Pb) the active mass is subjected to mechanical stress and expansion. Recrystallization processes take place with PbSO4 participation. During this process the mechanical stress decreases and the plate shrinks slightly. On battery charge PbSO2 crystals are reduced to Pb energetic crystals and the plate restores its initial thickness.
Changes in skeleton structure during cycling. During cycling the active mass loses its macrohomogeneity. During discharge the formation and growth of PbSO4 crystals occur in zones with easiest exchange of ion flows through the macropores. Consequently, mechanical stresses are concentrated in these zones, as a result of which the skeleton may break. Thus, the energetic parts of these zones are excluded from the charge and discharge reactions.
Through plate thickness measurements and SEM observations the effect of different types of expanders has been evaluated during cycling. It has been shown that expanders create stable NAM structure and delay the effect of “breathing” of NAM that causes gradual disintegration of the skeleton structure.
References
Keywords: plate expansion during cycling, negative active mass structure, NAM macrohomogeneity, energetic NAM structure, NAM skeleton break down, effect of NAM breathing, mechanical stress in NAM
